Ch. 3 Forces I. Newton’s 1 st Law: An object moving at a constant velocity keeps moving at that velocity unless an unbalanced net force acts on it (a.k.a. law of inertia) A. Newton’s 1 st Law: Perhaps more familiar: An object in motion tends to stay in motion, an object at rest tends to stay at rest, unless acted upon by an outside force B. Inertia: the tendency of objects to resist changes in their motion

Ch. 3 Forces Force is measured in N, but what are Newtons? 1 N is defined as the amount of force needed to give a 1 kg mass and acceleration of 1 m/s 2 Therefore: 1 N = 1 kg *m/s 2 C. Force: a push or a pull exerted on an object Movement depends on the net force (sum of all forces on the object)

Ch. 3 Forces II. Newton’s 2 nd Law: Different forces and masses affect the acceleration of objectsA. Newton’s 2 nd Law: 1. Greater Forces cause greater accelerations!(Directly related) 2. Greater masses cause smaller accelerations!(Inversely related) 3. Acceleration equals net force divided by mass( Equation: a = F net /m) 4. Example: A Force of 25 N on a mass of 10 kg. a = ? a. F net = 25 N, m = 10 kg, a = ? b. Equation: a = F net /m c. a = 25 N/10 kg d. a = 2.5 m/s 2

B. Forces opposing Motion 1. Friction: the force between two touching objects opposing motion a. Static friction: friction between two non-moving objects b. Sliding friction: friction between objects sliding past each other c. Rolling friction: friction with a rolling object and a surface 2. Air Resistance: the force opposing motion of objects moving through the air a. The amount depends on the object’s size, shape and speed b.Terminal Velocity: When the weight of a falling object and the upward force of air resistance are equal. Net force = 0, acceleration is 0, and speed is now constant!! Terminal Velocity Video

III. Gravity A. Law of gravity: There is a force of attraction between any two objects B. Due to inertia, all objects fall with the same acceleration (w/out air resistance) [9.8 m/s 2 ] C. Weight: Measurement of gravity’s pull on an object 1. The greater the gravity, the greater the weight of the object. 2. Weight is the force pulling on the object, mass is the stuff in the object. D. Orbiting 1. Objects in orbit float because all forces are cancelled 2. They have horizontal speeds so fast that they fall around the Earth 3.Centripetal force holds them in the circular motion with centripetal acceleration, but their speed keeps them from being pulled in! Orbiting Diagram Diagram – Mars Orbit Feather and hammer

Gravity:attractive force between any two objects A. Mass and distance affect the amount of gravity B. Weight (W):the measurement of gravity’s (g) pull on an object. D. Equation: W = mass x g (SI Units = Newtons (N)) C. g = acceleration due to gravity on Earth = 9.8 m/s 2 E. Example: If a person has a mass of 73 kg, what is their weight in Newtons? 2. W = mg 1. m = 73 kg g = 9.8 m/s2 W= ? 3. W = 73(9.8) 4. W = N

IV. Newton’s 3 rd Law of Motion A. Newton’s 3 rd Law:For every action force exerted on an object, that object will exert a reaction force back which is equal and opposite! 1.Action / Reaction forces are always on two separate objects, that is why they don’t cancel out. 2.Rockets use this law – the rocket pushes off of the fuel…and the fuel pushes off of the rocket “ Action” and “Reaction” are names of forces Forces ALWAYS occur in pairs Single forces NEVER happen Perhaps more familiar For every action, there is an equal and opposite reaction

“ Equal & Opposite ” In Newton’s 3 rd Law, “equal” means: Equal in size Equal in time. occur at EXACTLY the same time. In Newton’s 3 rd Law, “opposite” means: Opposite in direction forces are EXACTLY 180 o apart in direction.  Only forces that act on the same object can cancel.  Newton’s 3 rd Law action and reaction forces act on different objects, so they DON’T cancel.

Writing Action Reaction Forces ACTION Bowling ball hits the pin to the left. REACTION Pin hits the bowling ball to the right

Momentum B. Momentum:Inertia of an object (mass) in motion (velocity) 1. Momentum = mass x velocity (Equation: p = mv) [ SI units = kgm/s] 2.Example: An object with a mass of 25 kg and a velocity of 5 m/s. p = ? a. m = 25 kg, v = 5 m/s, p = ? b. p = mv c. p = 25 x 5 d. p = 125 kgm/s 3. Law of Conservation of Momentum:The momentum of an object cannot be lost or gained, but can be transferred to a different object in a collision… (pool balls)